1. Field of the Invention
The present invention relates to novel acid dianhydrides useful as a raw material for heat-resistant resins such as polyimides.
2. Description of the Prior Art
Because of their excellent heat-resistant properties, polyimides have been in wide use in various fields, ranging from the field of space and aeronautics and to the field of electronic communication. In recent years, however, the polymers are required, in addition to excellent heat-resistant properties, to have various characteristics suited for their particular uses.
For example, polyimides to be used as a base film for flexible printed circuit boards, as a carrier tape for TAB (tape automated bonding) or as a resin for laminated products are desired to have a low thermal expansion coefficient, a small dielectric constant and a low hygroscopicity. However, no polyimides have been known which satisfy these requirements to a satisfactory degree.
In order to obtain polyimides that satisfy the adove requirements, it is necessary to make the thermal expansion of polyimides quite low by making the main chain of polyimides as rigid as possible. A low thermal expansion can be readily attained when polyimides are synthesized by use of pyromellitic acid, which has the most rigid structure of all the existing compounds. In such a case, however, there is resulted an undesirably large polarization of imide groups, and hence it becomes impossible to attain low hygroscopicity. In theory, a low dielectric constant can be attained by introducing fluorine. However, the introduction of fluorine is disadvantageous in cost. In addition, it causes an undesirable lowering in the reactivity of resultant acid anhydrides.